Ami Mizuno was just a lonely, shy, child genius when she met Usagi Tuskino and her pet cat Luna that fateful day. After meeting those two, and being attacked by a Dark Kingdom youma, Ami faced her destiny and joined the senshi team as Sailor Mercury, warrior of water!
Ami is the smartest of the senshi, with an IQ of 300. She attends elite prep schools, such as Esai Juku (Albert Einstein School for the Gifted in the English dub), in addition to taking normal classes at Juuban Junior High (Crossroads Junior High in the English dub) with Usagi and Makoto. Ami is very devoted to her studies, and more often than not will put them before anything else. She also constantly lectures the other senshi (Mostly Usagi) about their grades. However Ami is a very shy and insecure girl, trying to please others and always wondering if she is doing well enough, even when others say she is far surpassing the standards. She is constantly full of doubts. As Sailor Mercury, however, Ami fights passionately and confidently, becoming a very valuable asset to the senshi team by figuring out enemy weaknesses using her Mini-Computer. Overall, she is shy and insecure about herself, but when put to the test, she is confident and a very good fighter.
In her past life, Ami was the princess of the planet Mercury, Princess Mercury. She was also one of Princess Serenity's four sailor guardians during the Silver Millenium. It is rumored that in the manga, she was in love with one of Prince Endymion's generals, Zoicite, before he took Queen Metallia's and Beryl's side against the Moon Kingdom. She fought to defend this kingdom from the attack, but died while protecting it. However, with Queen Serenity's power and the Silver Crystal, she was reborn on Earth with the Princess and other senshi.
During the reign of Neo-Queen Serenity and King Endymion in Crystal Tokyo, Ami is again one of the four Sailor guardians of the royal family. In the manga, during the attack by Nemisis, she was placed into a state of suspended animation along with the Neo-Queen, King, and other senshi. In the anime, she was one of the only ones left awake, and cast her energy with the other senshi to protect the royal castle and the sleeping queen. Luckily, she was rescued by Sailor Moon and Chibi-Usa, and life in Crystal Tokyo went back to normal.